Cancer-Associated Fibroblast-Like Tumor Cells Remodel the Ewing Sarcoma Tumor Microenvironment.

PURPOSE: Despite limited genetic and histologic heterogeneity, Ewing sarcoma (EwS) tumor cells are transcriptionally heterogeneous and display varying degrees of mesenchymal lineage specification in vitro. In this study, we investigated if and how transcriptional heterogeneity of EwS cells contributes to heterogeneity of tumor phenotypes in vivo. EXPERIMENTAL DESIGN: Single-cell proteogenomic-sequencing of EwS cell lines was performed and integrated with patient tumor transcriptomic data. Cell subpopulations were isolated by FACS for assessment of gene expression and phenotype. Digital spatial profiling and human whole transcriptome analysis interrogated transcriptomic heterogeneity in EwS xenografts. Tumor cell subpopulations and matrix protein deposition were evaluated in xenografts and patient tumors using multiplex immunofluorescence staining. RESULTS: We identified CD73 as a biomarker of highly mesenchymal EwS cell subpopulations in tumor models and patient biopsies. CD73+ tumor cells displayed distinct transcriptional and phenotypic properties, including selective upregulation of genes that are repressed by EWS::FLI1, and increased migratory potential. CD73+ cells were distinguished in vitro and in vivo by increased expression of matrisomal genes and abundant deposition of extracellular matrix (ECM) proteins. In epithelial-derived malignancies, ECM is largely deposited by cancer-associated fibroblasts (CAF), and we thus labeled CD73+ EwS cells, CAF-like tumor cells. Marked heterogeneity of CD73+ EwS cell frequency and distribution was detected in tumors in situ, and CAF-like tumor cells and associated ECM were observed in peri-necrotic regions and invasive foci. CONCLUSIONS: EwS tumor cells can adopt CAF-like properties, and these distinct cell subpopulations contribute to tumor heterogeneity by remodeling the tumor microenvironment. See related commentary by Kuo and Amatruda, p. 5002.

Carcinoma-associated fibroblast-like tumor cells remodel the Ewing sarcoma tumor microenvironment.

Tumor heterogeneity is a major driver of cancer progression. In epithelial-derived malignancies, carcinoma-associated fibroblasts (CAFs) contribute to tumor heterogeneity by depositing extracellular matrix (ECM) proteins that dynamically remodel the tumor microenvironment (TME). Ewing sarcomas (EwS) are histologically monomorphous, mesenchyme-derived tumors that are devoid of CAFs. Here we identify a previously uncharacterized subpopulation of transcriptionally distinct EwS tumor cells that deposit pro-tumorigenic ECM. Single cell analyses revealed that these CAF-like cells differ from bulk EwS cells by their upregulation of a matrisome-rich gene signature that is normally repressed by EWS::FLI1, the oncogenic fusion transcription factor that underlies EwS pathogenesis. Further, our studies showed that ECM-depositing tumor cells express the cell surface marker CD73, allowing for their isolation ex vivo and detection in situ. Spatial profiling of tumor xenografts and patient biopsies demonstrated that CD73 + EwS cells and tumor cell-derived ECM are prevalent along tumor borders and invasive fronts. Importantly, despite loss of EWS::FLI1-mediated gene repression, CD73 + EwS cells retain expression of EWS::FLI1 and the fusion-activated gene signature, as well as tumorigenic and proliferative capacities. Thus, EwS tumor cells can be reprogrammed to adopt CAF-like properties and these transcriptionally and phenotypically distinct cell subpopulations contribute to tumor heterogeneity by remodeling the TME.

ß-Catenin-Driven Differentiation Is a Tissue-Specific Epigenetic Vulnerability in Adrenal Cancer.

Adrenocortical carcinoma (ACC) is a rare cancer in which tissue-specific differentiation is paradoxically associated with dismal outcomes. The differentiated ACC subtype CIMP-high is prevalent, incurable, and routinely fatal. CIMP-high ACC possess abnormal DNA methylation and frequent ß-catenin-activating mutations. Here, we demonstrated that ACC differentiation is maintained by a balance between nuclear, tissue-specific ß-catenin-containing complexes, and the epigenome. On chromatin, ß-catenin bound master adrenal transcription factor SF1 and hijacked the adrenocortical super-enhancer landscape to maintain differentiation in CIMP-high ACC; off chromatin, ß-catenin bound histone methyltransferase EZH2. SF1/ß-catenin and EZH2/ß-catenin complexes present in normal adrenals persisted through all phases of ACC evolution. Pharmacologic EZH2 inhibition in CIMP-high ACC expelled SF1/ß-catenin from chromatin and favored EZH2/ß-catenin assembly, erasing differentiation and restraining cancer growth in vitro and in vivo. These studies illustrate how tissue-specific programs shape oncogene selection, surreptitiously encoding targetable therapeutic vulnerabilities. SIGNIFICANCE: Oncogenic ß-catenin can use tissue-specific partners to regulate cellular differentiation programs that can be reversed by epigenetic therapies, identifying epigenetic control of differentiation as a viable target for ß-catenin-driven cancers.

The importance of fusion protein activity in Ewing sarcoma and the cell intrinsic and extrinsic factors that regulate it: A review.

Accumulating evidence shows that despite clonal origins tumors eventually become complex communities comprised of phenotypically distinct cell subpopulations. This heterogeneity arises from both tumor cell intrinsic programs and signals from spatially and temporally dynamic microenvironments. While pediatric cancers usually lack the mutational burden of adult cancers, they still exhibit high levels of cellular heterogeneity that are largely mediated by epigenetic mechanisms. Ewing sarcomas are aggressive bone and soft tissue malignancies with peak incidence in adolescence and the prognosis for patients with relapsed and metastatic disease is dismal. Ewing sarcomas are driven by a single pathognomonic fusion between a FET protein and an ETS family transcription factor, the most common of which is EWS::FLI1. Despite sharing a single driver mutation, Ewing sarcoma cells demonstrate a high degree of transcriptional heterogeneity both between and within tumors. Recent studies have identified differential fusion protein activity as a key source of this heterogeneity which leads to profoundly different cellular phenotypes. Paradoxically, increased invasive and metastatic potential is associated with lower EWS::FLI1 activity. Here, we review what is currently understood about EWS::FLI1 activity, the cell autonomous and tumor microenvironmental factors that regulate it, and the downstream consequences of these activity states on tumor progression. We specifically highlight how transcription factor regulation, signaling pathway modulation, and the extracellular matrix intersect to create a complex network of tumor cell phenotypes. We propose that elucidation of the mechanisms by which these essential elements interact will enable the development of novel therapeutic approaches that are designed to target this complexity and ultimately improve patient outcomes.

EWS::FLI1 and HOXD13 Control Tumor Cell Plasticity in Ewing Sarcoma.

PURPOSE: Propagation of Ewing sarcoma requires precise regulation of EWS::FLI1 transcriptional activity. Determining the mechanisms of fusion regulation will advance our understanding of tumor progression. Here we investigated whether HOXD13, a developmental transcription factor that promotes Ewing sarcoma metastatic phenotypes, influences EWS::FLI1 transcriptional activity. EXPERIMENTAL DESIGN: Existing tumor and cell line datasets were used to define EWS::FLI1 binding sites and transcriptional targets. Chromatin immunoprecipitation and CRISPR interference were employed to identify enhancers. CUT&RUN and RNA sequencing defined binding sites and transcriptional targets of HOXD13. Transcriptional states were investigated using bulk and single-cell transcriptomic data from cell lines, patient-derived xenografts, and patient tumors. Mesenchymal phenotypes were assessed by gene set enrichment, flow cytometry, and migration assays. RESULTS: We found that EWS::FLI1 creates a de novo GGAA microsatellite enhancer in a developmentally conserved regulatory region of the HOXD locus. Knockdown of HOXD13 led to widespread changes in expression of developmental gene programs and EWS::FLI1 targets. HOXD13 binding was enriched at established EWS::FLI1 binding sites where it influenced expression of EWS::FLI1-activated genes. More strikingly, HOXD13 bound and activated EWS::FLI1-repressed genes, leading to adoption of mesenchymal and migratory cell states that are normally suppressed by the fusion. Single-cell analysis confirmed that direct transcriptional antagonism between HOXD13-mediated gene activation and EWS::FLI1-dependent gene repression defines the state of Ewing sarcoma cells along a mesenchymal axis. CONCLUSIONS: Ewing sarcoma tumors are comprised of tumor cells that exist along a mesenchymal transcriptional continuum. The identity of cells along this continuum is, in large part, determined by the competing activities of EWS::FLI1 and HOXD13. See related commentary by Weiss and Bailey, p. 4360.

Systemic Delivery of an Adjuvant CXCR4-CXCL12 Signaling Inhibitor Encapsulated in Synthetic Protein Nanoparticles for Glioma Immunotherapy.

Glioblastoma (GBM) is an aggressive primary brain cancer, with a 5 year survival of ∼5%. Challenges that hamper GBM therapeutic efficacy include (i) tumor heterogeneity, (ii) treatment resistance, (iii) immunosuppressive tumor microenvironment (TME), and (iv) the blood-brain barrier (BBB). The C-X-C motif chemokine ligand-12/C-X-C motif chemokine receptor-4 (CXCL12/CXCR4) signaling pathway is activated in GBM and is associated with tumor progression. Although the CXCR4 antagonist (AMD3100) has been proposed as an attractive anti-GBM therapeutic target, it has poor pharmacokinetic properties, and unfavorable bioavailability has hampered its clinical implementation. Thus, we developed synthetic protein nanoparticles (SPNPs) coated with the transcytotic peptide iRGD (AMD3100-SPNPs) to target the CXCL2/CXCR4 pathway in GBM via systemic delivery. We showed that AMD3100-SPNPs block CXCL12/CXCR4 signaling in three mouse and human GBM cell cultures in vitro and in a GBM mouse model in vivo. This results in (i) inhibition of GBM proliferation, (ii) reduced infiltration of CXCR4+ monocytic myeloid-derived suppressor cells (M-MDSCs) into the TME, (iii) restoration of BBB integrity, and (iv) induction of immunogenic cell death (ICD), sensitizing the tumor to radiotherapy and leading to anti-GBM immunity. Additionally, we showed that combining AMD3100-SPNPs with radiation led to long-term survival, with ∼60% of GBM tumor-bearing mice remaining tumor free after rechallenging with a second GBM in the contralateral hemisphere. This was due to a sustained anti-GBM immunological memory response that prevented tumor recurrence without additional treatment. In view of the potent ICD induction and reprogrammed tumor microenvironment, this SPNP-mediated strategy has a significant clinical translation applicability.

EWS-FLI1 and Menin Converge to Regulate ATF4 Activity in Ewing Sarcoma.

Ewing sarcomas are driven by EWS-ETS fusions, most commonly EWS-FLI1, which promotes widespread metabolic reprogramming, including activation of serine biosynthesis. We previously reported that serine biosynthesis is also activated in Ewing sarcoma by the scaffolding protein menin through as yet undefined mechanisms. Here, we investigated whether EWS-FLI1 and/or menin orchestrate serine biosynthesis via modulation of ATF4, a stress-response gene that acts as a master transcriptional regulator of serine biosynthesis in other tumors. Our results show that in Ewing sarcoma, ATF4 levels are high and that ATF4 modulates transcription of core serine synthesis pathway (SSP) genes. Inhibition of either EWS-FLI1 or menin leads to loss of ATF4, and this is associated with diminished expression of SSP transcripts and proteins. We identified and validated an EWS-FLI1 binding site at the ATF4 promoter, indicating that the fusion can directly activate ATF4 transcription. In contrast, our results suggest that menin-dependent regulation of ATF4 is mediated by transcriptional and post-transcriptional mechanisms. Importantly, our data also reveal that the downregulation of SSP genes that occurs in the context of EWS-FLI1 or menin loss is indicative of broader inhibition of ATF4-dependent transcription. Moreover, we find that menin inhibition similarly leads to loss of ATF4 and the ATF4-dependent transcriptional signature in MLL-rearranged B-cell acute lymphoblastic leukemia, extending our findings to another cancer in which menin serves an oncogenic role. IMPLICATIONS: These studies provide new insights into metabolic reprogramming in Ewing sarcoma and also uncover a previously undescribed role for menin in the regulation of ATF4.

A Bifunctional MAPK/PI3K Antagonist for Inhibition of Tumor Growth and Metastasis.

Responses to targeted therapies frequently are brief, with patients relapsing with drug-resistant tumors. For oncogenic MEK and BRAF inhibition, drug resistance commonly occurs through activation of PI3K/AKT/mTOR signaling and immune checkpoint modulation, providing a robust molecular target for concomitant therapy. Here, we evaluated the efficacy of a bifunctional kinase inhibitor (ST-162) that concurrently targets MAPK and PI3K signaling pathways. Treatment with ST-162 produced regression of mutant KRAS- or BRAF-addicted xenograft models of colorectal cancer and melanoma and stasis of BRAF/PTEN-mutant melanomas. Combining ST-162 with immune checkpoint blockers further increased efficacy in a syngeneic KRAS-mutant colorectal cancer model. Nascent transcriptome analysis revealed a unique gene set regulated by ST-162 related to melanoma metastasis. Subsequent mouse studies revealed ST-162 was a potent inhibitor of melanoma metastasis to the liver. These findings highlight the significant potential of a single molecule with multikinase activity to achieve tumor control, overcome resistance, and prevent metastases through modulation of interconnected cell signaling pathways. Mol Cancer Ther; 16(11); 2340-50. ©2017 AACR.

Structure-Guided Design and Initial Studies of a Bifunctional MEK/PI3K Inhibitor (ST-168).

The structure-based design of a new single entity, MEK/PI3K bifunctional inhibitor (7, ST-168), which displays improved MEK1 and PI3K isoform inhibition, is described. ST-168 demonstrated a 2.2-fold improvement in MEK1 inhibition and a 2.8-, 2.7-, 23-, and 2.5-fold improved inhibition toward the PI3Kα, PI3Kß, PI3Kδ, and PI3Kγ isoforms, respectively, as compared to a previous lead compound (4; ST-162) in in vitro enzymatic inhibition assays. ST-168 demonstrated superior tumoricidal efficacy over ST-162 in an A375 melanoma spheroid tumor model. ST-168 was comparatively more effective than ST-162 in promoting tumor control when administrated orally in a tumor therapy study conducted in an A375 melanoma mouse model confirming its bioavailability and efficacy toward combined in vivo MEK1/PI3K inhibition.

A transcriptomics approach uncovers novel roles for poly(ADP-ribosyl)ation in the basal defense response in Arabidopsis thaliana.

Pharmacological inhibition of poly(ADP-ribose) polymerase (PARP) or loss of Arabidopsis thaliana PARG1 (poly(ADP-ribose) glycohydrolase) disrupt a subset of plant defenses. In the present study we examined the impact of altered poly(ADP-ribosyl)ation on early gene expression induced by the microbe-associate molecular patterns (MAMPs) flagellin (flg22) and EF-Tu (elf18). Stringent statistical analyses and filtering identified 178 genes having MAMP-induced mRNA abundance patterns that were altered by either PARP inhibitor 3-aminobenzamide (3AB) or PARG1 knockout. From the identified set of 178 genes, over fifty Arabidopsis T-DNA insertion lines were chosen and screened for altered basal defense responses. Subtle alterations in callose deposition and/or seedling growth in response to those MAMPs were observed in knockouts of At3g55630 (FPGS3, a cytosolic folylpolyglutamate synthetase), At5g15660 (containing an F-box domain), At1g47370 (a TIR-X (Toll-Interleukin Receptor domain)), and At5g64060 (a predicted pectin methylesterase inhibitor). Over-represented GO terms for the gene expression study included "innate immune response" for elf18/parg1, highlighting a subset of elf18-activated defense-associated genes whose expression is altered in parg1 plants. The study also allowed a tightly controlled comparison of early mRNA abundance responses to flg22 and elf18 in wild-type Arabidopsis, which revealed many differences. The PARP inhibitor 3-methoxybenzamide (3MB) was also used in the gene expression profiling, but pleiotropic impacts of this inhibitor were observed. This transcriptomics study revealed targets for further dissection of MAMP-induced plant immune responses, impacts of PARP inhibitors, and the molecular mechanisms by which poly(ADP-ribosyl)ation regulates plant responses to MAMPs.